Polyoxometallates (POMs) respresent a large class of nanosized, polynuclear metal-oxo anions with a wide compositional and structural variety. POMs based on noble metal addenda could be attractive as catalysts and pre- catalysts of different chemical processes as well as precursors for nanoparticles synthesis.
We investigated e.g. how hydrogenation and oxidation influence the electronic and magnetic properties and how chemical pressure modifies the characteristic features of central 3d transition metal ions.
Selected publications:
Collaborations:
We investigate different kinds of oxide nanoparticles focussing on ferrite nanoparticles. Particularly we are interested in investigating the element-specific electronic and magnetic properties to tailor them e.g. for biomedical applications.
Magnetite nanoparticles. Magnetite (Fe3O4) nanoparticles are objects of intense research activities due to their broad range of applications covering technological, medical, and environmental applications. Magnetite is the Fe oxide with the highest net magnetic moment and crystallises in a cubic inverse spinel structure. In a simple picture, it consists of Fe2+ ions on octahedral lattice sites, Fe3+ on octahedral and Fe3+ on tetrahedral lattice sites. Although it is well-known to date that due to hybridisation effects the charges of the ions differ from the nominal values, this notation is still used to distinguish between the different Fe species.
We studied the influence of the surface on the magnetic properties like spin and orbital magnetic moments, spin canting, and the so-called Verwey transition, a phase transition that occurs in pure magnetite only. Parts of our results are supported by DFT calculations performed by Soumyajyoti Haldar and Sumanta Bhandary at the Uppsala University.
Cobalt ferrite nanoparticles. Co ferrite (CoFe2O4, CFO) is similar to magnetite described above, but with 50% of the octahedral lattice occupied by Co2+ ions while all Fe ions have a nominal valence state of +3. In real samples, cation disorder is common, which means that some of the Co2+ ions are located on tetrahedral lattice sites. Besides its technological relevance e.g. because of large magnetostriction (see also next paragraph about mutiferroic nanocomposites), CFO nanoparticles are interesting candidates for biomedical applications as well due to their high physicochemical stability.
By decoration with Pd nanoparticles, we improved the performance for future magnetic hyperthermia cancer treatment and studied in detail, how the Pd influences the magnetic properties of CFO.
Selected publications:
Multiferroic materials showing both magnetic and electric ordering allow an additional degree of freedom in the design of actuators, transducers and storage devices and thus have attracted scientific interest from the technological perspective as well as from basic research. Because the choice of single-phase multiferroic materials being suitable at room temperature is limited, the use of magnetoelectric two-phase composites has proven to be more promising.
In our work, we studied nanocomposites of ferroelectric BaTiO3 (BTO) and ferrimagnetic CoFe2O4 (CFO). By pulsed laser deposition (PLD) CFO nanopillers in a BTO matrix -a so-called (1,3)-type composite - were grown epitaxially by Dr. Pavel Borisov in the group of Prof. W. Kleemann. Starting from CFO and BTO nanoparticles, ceramic samples of CFO grains in a BTO matrix ((0,3)-type) as well as BTO grains in a CFO matrix ((3,0)-type) were obtained by sintering and annealing in the group of Prof. D.C. Lupascu. The coupling between CFO and BTO was studied by x-ray absorption spectroscopy at the Ti and Co L3,2 absorption edges and its linear and circular dichroisms. For the epitaxial (1,3)-type composite we were able to demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the structures and discussed it in terms of off-diagonal magnetostrictive-piezo-electric coupling. This coupling creates staggered in-plane components of the electric polarisation, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BTO matrix.
By comparison of (0,3)- and(3,0)-type composites we showed that the latter exhibits improved magnetic properties while the good ferroelectric characteristics were retained. In addition, a good qualitative agreement between the magnetic field dependence of the electric polarisation obtained from XLD with measurements of the electrically induced magnetization using ac-SQUID susceptometry was obtained.
Selected publications:
Our research in the field of metallic nanoparticles focusses on Fe-based systems like pure Fe nanoparticles, FePt, and FeRh nanoparticles. All were wet-chemically synthesised by colleagues from chemistry groups, in particular Frédéric Pelletier and Diana Ciuculescu from the group of Prof. Catherine Amiens at the LCC Toulouse and Prof. Shouheng Sun. We used the x-ray absorption spectroscopy to determine structural and magnetic properties.
Fe nanoparticles. Fe nanoparticles can be used as catalysts, e.g. for the Haber-Bosch process and in Fischer-Tropsch reactions. By analysis of the extended x-ray absorption fine structure (EXAFS), boron incorporations in Fe nanoparticles were identified and quantified. With this results, the effectiveness of amine-borane as reducing agent for the synthesis of iron nanoparticles has been investigated. Furthermore, the reactivity of amine-borane and amino-borane complexes in the presence of pure Fe nanoparticles was studied.
FePt nanoparticles. FePt in its chemically orderes phase has a very high magnetocrystalline anisotropy that stbilises the magnetisation direction even at the nanoscale. Thus, ensembles of FePt nanoparticles are the prime candidate for ultra-high density magnetic storage media. In our work, we studied the possibility to tailor the magnetic properties of FePt nanoparticles by capping. For instance, experimental results on oxide-free FePt nanoparticles embedded in Al were compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allowed us to present three rules to achieve desired magnetic properties for various applications.
FeRh nanoparticles. The 4d metal Rh almost fulfills the Stoner criterion for the occurence of ferromagnetic order. In contact to ferromagnets, it can be easily spin polarised and exhibits sizeable magnetic moments. In the group of Prof. Catherine Amiens at the Laboratoire de Chimie de Coordination, core/shell nanoparticles of Fe and Rh were synthesised. By analysis of the extended x-ray absorption fine structure (EXAFS), we studied the quality of the core/shell structure and used the results to interpret the magnetic moments of Fe and Rh atoms as determined by x-ray magnetic circular dichroism (XMCD) for different synthesis methods.
Pd nanoparticles. Commercially available Pd nanoparticles are investigated by x-ray absorption spectroscopy to study the influence of hydrogen loading on structural and electronic properties. Since the Pd absorption edges accessible in the soft x-ray regime are close to the oxygen K absorption egde, this is an additional challenge from the technical perspective.
Selected publications: Collaborations:
Auger Electron Spectroscopy is a surface sensitive method to analyse the chemical composition of a sample. Under bombardment with high-energetic electrons (usually 3-5 keV), Auger electrons are emitted. Their energies are determined only by the differences between energy levels involved during the excitation and relaxation process and are a fingerprint of the chemical species.
Analysis of the EXAFS is a method to investigate the local environment around an absorbing element. In a simple picture, the EXAFS signal is an interference effect between the outgoing photo-electron excited by x-ray absorption and its backscattered waves from neighbouring atoms. The frequency of the EXAFS oscillations depends mainly on the distance of nearest neighbour atoms, while the envelope contains information about the atomic species and coordination number.
The analysis of EXAFS data can be performed using a standard Fourier analysis e.g. using the Artemis software [1] based on the FEFFIT [2] programs. After definition of the structure, the EXAFS signal is calculated for various scattering paths and summed up to fit the measured data.
It can be complemented by an analysis of the wavelet transform of EXAFS oscillations. The wavelet transform can have high resolution in both real space and reciprocal space and visualises contributions to the EXAFS signal from different backscattering elements without further data treatment. We adopted the AGU-Vallen Wavelet package which has been developed by Vallen Systeme GmbH, Aoyama Gakuin University (AGU), Tokyo, and University of Denver for analysis of acoustic emission.
Selected publications: Related links:
The diffraction pattern of low-energy electrons (20 - 200eV) on a fluorescent screen is a map of the sample surface in reciprocal space and can be used to determine the symmetry and in-plane lattice constants of the surface. A more accurate determination of atomic positions at the surface including information about the out-of-plane lattice constant can be obtained by measuring the energy-dependent intensity of the diffraction spots (I-V LEED) and compare it to calculated I-V curves.
In x-ray absorption spectroscopy, a core-level electron is excited into a higher unoccupied state, if the photon energy matches the energy needed for the allowed transition. These so-called absorption edges are element-specific and depend also on the chemical environment. The absorption intensity is a measure of the number of unoccupied final states and the shape of the absorption signal in the near-edge region contains information about the density of unoccupied states. The absorption can be measured either in transmission for the case of thin samples, by measuring the fluorescence yield or the total electron yield by detecting the sample drain current. We could show that it is even possible to obtain soft-xray XANES signals from nanoparticles in an organic solvent.
It may be useful to compare experimental data to simulated absorption spectra, e.g. to unveil the origin of spectral faetures. For the case of metal ions, the CTM4XAS charge-transfer multiplet software based on a Hartree-Fock method is known to give reliable results. For other (metallic) systems, band structure calculations are performed e.g. using the spin polarised relativistic Korringa-Kohn-Rostoker (SPR-KKR) package. While calculations for bulk materials with a high symmetry can be performed on a common office PC, support is appreciated for calculations e.g. for clusters or other low-symmetry systems.
Selected publications: Related links:
In the case of absorption of circularly polarized x-rays, the excited core-level electrons are polarised regarding both their angular momentum and spin. This polarisation is reversed for reversed polarisation of x-rays. In a simple two-step model, the spin-polarised electrons probe the spin polarisation of the unoccupied final states yielding a large absorption intensity for the case of matching polarisation and a small absorption intensity otherwise, i.e. the so-called circular dichroism. The XMCD intensity is proportional to the spin polarisation of unoccupied final states (magnetisation) and can be used to deduce spin and orbital magnetic moments.
By measuring the x-ray absorption of linearly polarised light for different angles of incidence, one can use the electric field vector of the x-rays as a search light for the maximum and minimum unoccupied states. Thus, with XLD one can probe anisotropies in the charge density that may result from the crystal structure (x-ray natural linear dichroism) or magnetic ordering (x-ray magnetic linear dichroism).
The development of improved hydrogen storage concepts is crucial for the advancement of hydrogen and fuel cell technologies in applications including stationary power, portable power, and transportation. Since hydrogen has the highest energy per mass of any fuel, but has a low density under ambient conditions, advance storage methods are needed to obtain a sufficient energy density. To date, most research into hydrogen storage is focussed on storing hydrogen as a lightweight, compact energy carrier for mobile applications.
Pd is an archetypical hydrogen storage metal and an effective catalyst for hydrogen-related reactions in a variety of industrial processes and stores hydrogen under ambient conditions. The interaction between hydrogen and Pd is relevant not only from the perspective of basic research, but also for applications in hydrogen storage technology. In the bulk material, the hydrogen occupies interstitial lattice sites of the face-centred cubic Pd forming two different PdHx phases. It has been found that the hydrogen loading saturates at about x = 0.7, At this concentration, the Pd lattice is already expanded by a value of about 6%. For Pd nanoparticles, even larger lattice constants were reported and discussed in terms of an enhanced hydrogenation of nanoparticles with respect to thin films or bulk materials due to additional surface and subsurface adsorption sites.
Further reading:
Magnetic nanoparticles can be used for many medical applications, e.g. cell labelling, targeted drug delivery, as contrast agents in magnetic resonance imaging (MRI) or in hyperthermia cancer treatment. For all these applications, the nanoparticles have to biocompatible like e.g. the Fe oxide nanoparticles we investigate.
MRI contrast agents. Magnetic resonance imaging (MRI) is based on nuclear magnetic resonance (NMR) which describes the resonant absorption of an alternating magnetic field applied perpendicularly to a static magnetic field. Resonant absorption occurs if the frequency of the alternating magnetic field equals the Larmor precession frequency of the nuclear magnetic moments. For MRI, usually NMR of hydrogen nuclei, i.e. protons, is used because hydrogen is present in all biological tissues.
The macrospin of the nanoparticles is connected to a large magnetic stray field that alters the relaxation time of the protons in the nearest environment. In general, there three different relaxation times exist: T1, T2 and T2*. The longitudinal relaxation time T1, is the decay constant for the recovery of the nuclear spin magnetisation component along the direction of the external magnetic field towards its thermal equilibrium value. It is commonly called spin-lattice relaxation since it involves the exchange of energy with its surroundings. The transverse relaxation time T2 is the decay constant for the magnetisation component perpendicular to the external field and corresponds to a phase decoherence of the transverse nuclear spin magnetisation. Since T2 is affected only by the dynamics of the nuclear spins, it is called spin-spin relaxation time. Magnetic field inhomogeneities yield a distribution of resonance frequencies resulting in a dephasing of nuclear spins as well. The latter decay constant is denoted T2*. The use of magnetic nanoparticles as contrast agents indirectly influences the T2 and T2* relaxation. This gives rise to a higher contrast in the spin echo MRI which is used as a method to visualise the transverse relaxation behaviour. Besides biocompatibility, a high net magnetic moment of the nanoparticles is an important requisite for a high MRI contrast. Therefore, magnetite (Fe3O4) nanoparticles are favoured over maghaemite (γ-Fe2O3) as contrast agents.
Hyperthermia. The cancer treatment using hyperthermia was already successfully tested for functionalised Fe oxide nanoparticles and is approved in the EU for the treatment of brain tumours. The iron oxide nanoparticles - γ-Fe2O3 (maghaemite) or Fe3O4 (magnetite) - can be injected directly into a tumour similar to a biopsy procedure, injected into the arterial supply of tumour tissue, and/or it will be enriched at tumour sites by an appropriate antibody-conjugation. The latter is advantageous, if the hyperthermia treatment must be repeated, while the direct injection is usually connected to a higher local concentration of nanoparticles.
By application of an alternating magnetic field, the particles generate heat that may destroy the surrounding cancer cells or support chemotherapies where already a moderate tissue heating leads to a more effective cell destruction.
Further reading:
A new application of magnetite (Fe3O4) nanoparticles is the removal of heavy metals or actinides from waste water. The conventional way to clean waste water from metal ions is by the formation of metal hydroxides that usually have a low solubilities. However, the metal hydroxides can form gelatinous precipitates, which are difficult to filter and more chemicals have to be added frequently to facilitate the filtration process.
To clean the waste water using nanopartilces instead, small seeds of magnetite nanoparticles are dispersed in the water. After adding additional Fe2+ ions, ferrites start to grow that include the heavy metal or actinide ions. It was shown, that a large variety of ions can be incorporated in one step, e.g. Ag, As, Cd, Co, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sb, Ti, U, V, W, Zn, and Zr without noticeable removal of natural components like Ca and Mg ions. When the growth process is finished, the ferrite nanoprticles can be easily removed in one step from the water. This can be done by conventional filtration, because the ferrites are small crystalline solids and do not form gelatinous precipitates as it can happen in the conventional removal process. Another possibility is to remove the ferrites by the so-called magnetic filtration. By application of a magnetic field, the ferrite nanoparticles are concentrated and can be easily separated from the clean water. In contrast to common filtration, this method works also for very small nanoparticles.
Further reading:
Magnetic data storage. One idea to further increase the data storage density is to represent a bit by the magnetisation direction of one nanoparticle that is much smaller than the bit size to date. This approach gives the possbility to reach a storage density in the range of Tbit/cm2. However, to achieve a magnetisation that is stable over ten years as desired for data storage applications, materials with a high magnetocrystalline anisotropy is needed that enhances the energy barrier to switch the magnetisation direction. FePt is one of the materials with the highest known anisotropy and is a prime candidate for magnetic data storage using nanoparticle ensembles.
The use of FePt nanoparticles as magnetic storage media has been discussed for more than a decade. However, there are still several obstacles that have to be overcome. In particular, there seems to be a reduced anisotropy in the nanoparticles with respect to the corresponding bulk material, the arrangement of nanoparticles in dense regular superlattices over large areas is not satisfactorily solved and the alignment of the easy axes of magnetisation is another delicate task.
Magnetoelectric data storage. If the magnetic nanoscale bits are made of a magnetostrictive material like CoFe2O4 and embedded in an electrically insulating piezoelectric matrix like BaTiO3, this device could be used as an energy saving data storage device, in which the bits can be written be application of a voltage without any electric current flowing.
Further reading: